Gas-phase polymerization processes, such as the UNIPOL™ PP process of The Dow Chemical Company, are recognized as one of the most economical methods of manufacturing various polyolefin products, e.g., polyethylene, polypropylene, etc. In the gas-phase process, a high-activity catalyst is usually fed into a fluidized-bed reactor in the form of very small particles, and monomers (and comonomers if present) are converted to a polymer that grows on the catalyst particles. From the production viewpoint, such a polymerization process is desired to run at operating conditions which include (but not limited to):                (A) High monomer partial pressure for increasing catalyst productivity and thus reducing the operational cost of the process. The catalyst productivity is proportional to the partial pressures of the monomer and comonomer(s). For propylene-based polymers, the main monomer is propylene, and the comonomers can be ethylene, butene and other alpha olefins. The comonomer partial pressure is determined by the comonomer/propylene ratio in the reactor, which is related to the desired properties of the polymer.        (B) High reactor temperature for increasing catalyst productivity, reducing external donor usage (this is because the catalyst's xylene-solubles response is better under relatively high temperature), and improving some polymer properties, such as achieving relatively narrower molecular weight distribution. In addition, high reactor temperature increases the overall cooling capacity which can otherwise limit total throughput.        
The gas-phase process for making a propylene-based polymer in a fluidized-bed reactor containing a fluidized-bed comprising catalyst particles is well known in the art. One well known commercial process is the UNIPOL® Polypropylene process available for license by W.R. Grace & Co.-Conn. and/or its affiliates. U.S. Pat. No. 4,543,399 discloses that the polymer-forming reaction is exothermic, making it necessary to maintain in some fashion the temperature of the gas stream inside the reactor at a temperature not only below the resin and catalyst degradation temperatures, but also below the fusion or sticking temperature of resin particles produced during the polymerization reaction. This is necessary to prevent plugging of the reactor due to the polymer particle agglomeration and rapid growth of polymer chunks. U.S. Pat. No. 6,460,412 discloses that the operating temperature of the fluidized-bed polymerization reactor is generally ranging from 10° C. to 150° C., preferably 40° C. to 125° C.
In the literature, there are correlations of the polymer melting temperature and crystallization temperature (Tc) with the ethylene comonomer content for specific type of polymers (e.g., FIGS. 6.1.12-6.1.15 of Julie Tammy Uan-Zo-li, PhD Dissertation of Virginia Polytechnic Institute and State University, September 2005). However, those correlations are not generally applicable to a wide range of polymers and comonomer contents (such as terpolymer), and more importantly, those correlations are not related to the maximum allowed reactor temperature of terpolymer or butene copolymer operation in a gas-phase fluidized-bed reactor. Moreover, if propylene partial pressure and/or reactor temperature is too high, significant polymer particle agglomeration is observed, thereby resulting in reactor upsets.
High comonomer content ethylene-butene-propylene terpolymers (EBPT) and butene-propylene random copolymers (BPRCP) are attractive polymers because of their unique product properties and end-use applications. For example, the fast line speed of polymer-film making is critical for competitiveness in food packaging. The high-comonomer-content EBPT and BPRCP polymer materials are able to achieve a higher line speed at the same film fabrication condition. Also the polymer materials can be melted within a shorter time duration, thanks to the relatively lower melting temperature.
The production of these high comonomer (i.e., ethylene and butene) content polymers, however, can be difficult in gas-phase reactors. The copolymer content is limited by the polymer particle stickiness which can result in particle agglomeration, and eventually forming polymer “chunks” and “sheets,” which often force reactor shutdowns.
When the monomer and/or comonomer partial pressure is increased, each local active site in the catalyst generates more heat, and it is more likely to have the polymer particle stickiness concerns because of the limit in heat transfer. An insufficient heat transfer can result in the heat accumulation and the softening or even melting of the polymer, hence the particle agglomeration. Also, above a certain monomer partial pressure, the morphology of the particles changes to an open “popcorn-like” particle shape. This type of morphology is undesirable and effectively limits the monomer's and comonomers' partial pressures during the operation of manufacturing random copolymers. Regarding the reactor temperature, although a fluidized-bed reactor has excellent temperature uniformity, a bed temperature close to the softening temperature of the product would likely cause polymer stickiness, and hence negatively affect the reactor operability.
Because of these difficulties, high comonomer content EBPT and BPRCP products are rarely produced in gas-phase reactors, especially in stirred gas-phase reactors where the gas velocity is relatively low and the bed material's mixing is relatively less vigorous. Instead, multiple reactors in series, often running under different multi-phase conditions, and often with a pre-polymerization step, are used. For example, one commercial process uses a slurry loop reactor and a gas-phase reactor together to make terpolymers (e.g., U.S. Pat. No. 6,455,643). Such complex multi-reactor setups require relatively high investment and operation cost because of the reactor system.
Even with a fully fluidized polypropylene reactor, product grades with high butene and ethylene contents cannot be made without significantly jeopardizing reactor operation, such as being limited to low monomer partial pressure and/or low operating temperature. These types of reactor conditions result in low catalyst productivity and significantly reduced cooling capacity and thus reduce reactor throughput capacity.